1. Field of the Invention
This invention relates to continuously variable transmissions having a primary variable pulley system and a secondary variable pulley system to control the transmission ratio between the input and output shafts. More particularly, this invention relates to a double acting servo of the secondary variable pulley system of a continuously variable transmission which provides greater clamping forces at lower transmission ratios.
2. Description of the Prior Art
A continuously variable transmission (CVT) utilizes a pair of adjustable or variable pulleys mounted on a pair of shafts, and an endless belt intercoupled therebetween, to transmit torque from an input source, such as an engine, to an output, such as a vehicle driveline. Each pulley has at least one sheave that is axially fixed and another sheave that is axially movable relative to the first. A flexible belt of metal or elastomeric material interconnects the pulleys.
The inner faces of the sheaves of the pulleys are bevelled or chamfered so that movement of the axially displaceable sheave relative to the fixed sheave adjusts the distance between the sheaves and, thus, the effective pulley diameter. The first pulley, or primary pulley, is mounted on a primary or input shaft and is directly driven by the engine, or is driven by the engine through a torque converter, fluid coupling or start clutch. The second pulley, or secondary pulley, is mounted on a secondary or output shaft and the output of the secondary shaft drives the drivetrain of the vehicle. The drive train may be connected to the secondary shaft through a clutch. U.S. Pat. No. 4,433,594, entitled "Variable Pulley Transmission", provides further information regarding CVTs and is incorporated herein by reference in terms of background to the present invention.
The primary and secondary variable pulley systems include servo systems for movement of the pulley sheaves. The displaceable sheave of each pulley includes an annular chamber for receiving fluid to move the sheave and thus change the effective pulley diameter. Increasing the fluid in the chamber increases the effective diameter of the pulley. A fluid is exhausted from the chamber, the pulley diameter is decreased. The effective diameter of the primary pulley is moved in one direction as the effective diameter of the secondary pulley is moved in the other direction.
The movement of the sheave of the primary pulley servo regulates the transmission ratio across the CVT. The movement of the sheave of the secondary pulley servo regulates the clamping force on the belt connecting the primary and secondary pulleys. Sufficient clamping force is necessary to prevent damage that might result from slippage of the belt. The present invention is directed primarily to providing sufficient clamping force at the secondary pulley servo to prevent belt slippage.
The provision of sufficient clamping force to the secondary sheave becomes especially difficult in high torque applications. In such applications, certain designs of conventional CVT's may not be sufficient to supply the necessary range of vehicle drive ratios or a sufficient amount of output torque. For example, in a motor vehicle transmission, a single two-sheave CVT may not be capable of accomplishing all four of the following objectives: (a) providing the high torque necessary or desirable to optimally launch the vehicle from a stationary position without causing the belt to slip on the secondary sheave, particularly when the vehicle is facing up a steep incline or pulling a trailer; (b) providing a low enough transmission ratio to deliver a sufficiently high starting torque to allow the vehicle to move very slowly at a practical engine speed; (c) providing a high enough transmission ratio that the vehicle can travel efficiently at a high rate, as on an interstate highway; and (d) counteracting the undesired centrifugal pressure increase in the rotating hydraulic sheave cylinders which is observed at high speeds. These factors are explained in more detail below.
The need for high wheel torque, or a low starting transmission ratio, at launch, is complicated by the low torque supplied by the engine at low engine speeds. To maximize the launching torque supplied by the engine, the torque from the engine must be multiplied at low speeds by inserting a conventional torque converter between the engine output and CVT input. Alternatively, the CVT transmission ratio must be excessively low at low speeds, which consequently limits the upper limit of the high-speed transmission ratio.
The use of a torque converter overcomes these low torque and low-speed transmission ratio problems without compromising the high ratio. The torque converter has no multiplication effect on the input or output torque at high ratios, as the torque converter is effectively locked up at high ratios. In certain applications, the output of the CVT alone may not be adequate to provide sufficient launch of the vehicle. Unfortunately, the torque multiplication provided by the torque converter increases the secondary clamping force required to prevent the belt from slipping.
The output torque of the CVT and the effective diameter of the primary sheave thus determine the necessary clamping force at the secondary sheave to prevent slipping of the belt. At vehicle launch, a higher output torque necessitates a higher clamping force. Moreover, at vehicle launch, the effective diameter of the primary sheave is at its smallest position. Therefore, the required secondary servo clamping force is highest at launch. To increase the force at the secondary servo, the area of the servo chamber for force application can be increased, or the pressure of the fluid applied to the chamber can be increased.
An important consideration in sizing the secondary servo for the increased clamping force required at low ratios, is the effect of the design on the centrifugal forces generated by the oil rotating with the servo. The centrifugal force generated by the oil increases with the square of the angular velocity of the sheave and the fourth power of the sheave radius. The centrifugal force of the oil on the sheave servo acts to increase the clamping force on the belt and can cause excessive clamping forces at high secondary angular velocities.
The traditional method of minimizing the negative effects of the centrifugal oil pressure is to centrifugally balance the servo. This is typically accomplished by using a single secondary with a balance can adjacent to the servo to provide a counteracting centrifugal force. This can is kept filled with oil either through an orifice in the apply servo or from an external source. The can is typically open to atmosphere and only acts to reduce the belt clamping force through the centrifugal force of the oil contained within its volume.
High secondary forces due to centrifugal effects not only cause excessively high clamping forces on the belt which contribute to transmission inefficiency; such forces can make it difficult or impossible to obtain the required primary to secondary belt clamping force ratio necessary to maintain a desired ratio. Thus, if the force on the secondary is at a high level, the force on the primary must be that much higher to obtain the necessary force ratio.
Increased clamping force can be obtained by increasing the pressure of the hydraulic fluid. Since the hydraulic fluid of the secondary servo is pressurized by a pump, the fluid to the servo can be supplied at a very high pressure during launch to provide the necessary clamping force. The fluid pressure can then be moderated at higher angular velocities, particularly as the fluid becomes self-pressurized by centrifugal force.
Unfortunately, this solution is not practical in some applications. First, the pump size must be impractically large to produce a pressure of sufficient magnitude to provide the necessary clamping force at launch to a servo piston of moderate diameter. Second, even if a suitable pump is provided, the pressure which can be delivered to the servo is limited by the pressure capacity of the seals of the secondary shaft. Moreover, in a CVT which utilizes a torque converter, the diameter of a single secondary servo that is necessary to obtain the required apply area to operate with reasonable pressures is so large as to cause centrifugal problems.
The present invention is an attempt to provide a CVT capable of supplying a high output torque at launch while overcoming the above-described problems. While CVTs with double-acting primary servos are known in the prior art, the present invention utilizes a double-acting secondary servo in an attempt to address the above-described problems.